We just bought a typical 4 floor split level, that we would like to convert to a contemporary. In phase 1 we would like to raise the roof on the living space from 8' to 12' and also make a 22x10 addition in the rear. Current dimension is 22x25. Open floor plan in the 22x35' space will include living room, dining room and kitchen. Since there is no floor above this roof, is it possible to take the roof off, extend the walls by 4' and put on a new roof? Or do we have to build new walls. The roof would be changed from a gable roof to flat roof.

What is the existing wall framing? Wood Stud, 2x4 at 16" o.c.? Masonry? Light-gage metal studs? Typically it is a major change to the structure and stresses to raise a roof 4'-0". It can be done with extending the existing walls, it just really depends on the material of the walls. You might want to get a structural engineer to come out and render an opinion.